The Institute of Electrical and Electronics Engineers (IEEE) has adopted a set of standards for wireless local area networks (LANs), known as 802.11. Wireless products satisfying 802.11a, 802.11b, and 802.11g are currently on the market, for example.
Recently, an 802.11n standard, known also as the Enhancement for High Throughput wireless standard, has emerged. Under the 802.11n standard, transmitters and receivers each have multiple antennas for transmission and reception of data. As a multiple input, multiple output (MIMO) technology, 802.11n is designed to coordinate multiple simultaneous radio signals, and is expected to support a bandwidth of greater than 100 megabits per second (Mbps). In addition to MIMO, the 802.11n standard includes other features to increase throughput. (The 802.11n D2.0 (TGn) is an official draft of the IEEE Standards Association 802.11 Wireless LAN Working Group, published on Feb. 7, 2007.)
A single basic service set (BSS) of a wireless local area network (LAN) may include an access point (AP) and a number of different stations (STA). The wireless LAN may include a second BSS, a third, and so on. Entities in each BSS may communicate at any of a number of rates (known as the basic rate set). The entities of the BSS (the APs and STAs) communicate using frames, control frames, management frames, or data frames. Generally speaking, frames transmitted at a lower rate will travel farther, e.g., have a longer range.
Some stations may benefit from operating in an extended range, even if the extended range is available only at low rates. Several methods have been suggested to extend the range as part of the 802.11 network. Some of them (the TGn standard) use various techniques to allow the current basic orthogonal frequency division multiplexing (OFDM) structure, which is limited to a rate of 6 Megabits per second (Mbps), to use lower rates. These techniques enable operations in lower signal to noise ratios (SNRs) and thus higher ranges.
Among the techniques suggested for range extensions are: 1) Space Time Block Codes (STBC); 2) low-rate binary convolutional codes (BCC) coding (at ¼rate); 3) duplicating data in the frequency domain; 4) duplicating data in the time domain; 5) using 128-point fast Fourier transform (FFT) on an 8 microsecond (usec) symbols with 1.6 guard interval to combat larger delay spread. All of these techniques improve the reception of the data part of the packet, but do not improve the acquisition and packet detection.
In the current 802.11a/g/n specifications, detection is performed on an 8 usec signal. Several processes, including automatic gain control (AGC) setting and coarse frequency acquisition, are based on the 8 usec signal, limiting the time allowed for acquisition. The acquisition already limits the performance of range extension methods that exists today (such as STBC), and may limit the performance of multiple-antenna receivers as well.
802.11n devices may operate in one of three modes: legacy, mixed, and Green Field. In the legacy mode, the 802.11n device operates just like an 802.11a, 802.11b, or an 802.11g device. In the mixed mode, the 802.11n device operates either as an 802.11n device or as a legacy (802.11a,b,g) device. In the Green Field mode, the 802.11n device operates only according to the 802.11n standard. The 802.11n device achieves its highest throughput in the Green Field mode.